Usually, when image data or audio data is stored or transmitted, encoding (or compression) is carried out to decrease an amount of the data. One of various encoding techniques that have been proposed is differential pulse code modulation (DPCM) coding.
A common DPCM encoding apparatus calculates a difference between an input pixel and a predicted pixel, and quantizes the difference, and then, performs sequential encoding on the quantized value. The encoding apparatus dequantizes the difference and adds the value with the predicted pixel to generate a local decoded pixel. Further, the encoding apparatus uses the local decoded pixel as a reference pixel to generate a predicted pixel for the next input pixel. In other words, the encoding apparatus performs predictive encoding of the next input pixel only after completing difference calculation, quantization and dequantization of a current input pixel, generation of a reference pixel and generation of a predicted pixel for the next pixel. In the following explanation, the series of processes will be referred to as an encoding loop. When a pixel is synchronized with the clock and input to the encoding apparatus, the higher the input rate is, the shorter the clock period becomes. As a clock period becomes shorter, it becomes difficult to complete the encoding loop in one clock period, and thus, a plurality of clock periods will be required to complete one encoding loop. In other words, a rate of pixel encoding will be lower than a rate of inputting pixels. In this situation, the encoding apparatus may decrease data throughput of a system, and may be a bottleneck for communication.
JP-A 04-225694 (KOKAI) suggests a technique for increasing speed of data processing for a DPCM encoding apparatus. More specifically, the encoding apparatus disclosed in JP-A 04-225694 (KOKAI) also calculates a difference between an input pixel and a predicted pixel, quantizes the difference, and performs sequential encoding on the quantized value. In contrast, the encoding apparatus dequantizes the difference, adds the dequantized difference to the predicted pixel, and generates the predicted pixel for the next input pixel. In other words, according to the encoding apparatus of JP-A 04-225694 (KOKAI), the generation of a reference pixel and the generation of a predicted pixel for the next pixel in the above-described encoding loop can be combined.